Biology • Year 12 • Module 6 • Lesson 3
Point Mutation — Base-Level Genetic Change
Lock in the vocabulary of substitution / insertion / deletion, the silent–missense–nonsense distinction, and the codon-frame logic that drives frameshift effects.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: point mutation, substitution, insertion, deletion, frameshift, silent mutation, missense mutation, nonsense mutation, codon, reading frame. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | A mutation affecting one base pair, or a very small number of bases, in the DNA sequence. | |
| 1.2 | A point mutation in which one base in the DNA is replaced by a different base. | |
| 1.3 | A point mutation in which one or more bases are added into the DNA sequence. | |
| 1.4 | A point mutation in which one or more bases are removed from the DNA sequence. | |
| 1.5 | A shift in the codon reading frame caused by an insertion or deletion of bases not in multiples of three. | |
| 1.6 | A substitution that does not change the amino acid coded for, because the new codon still specifies the same amino acid. | |
| 1.7 | A substitution that changes one codon so it codes for a different amino acid. | |
| 1.8 | A substitution that changes a codon to a stop codon, prematurely truncating the polypeptide. | |
| 1.9 | A triplet of three bases in mRNA that specifies a particular amino acid (or a stop signal). | |
| 1.10 | The grouping of bases into three-base codons used by the ribosome during translation. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version. 8 marks (1 for T/F, 1 for the correction where needed)
2.1 Every point mutation must change the amino acid sequence of the protein it codes for. T / F
2.2 An insertion or deletion of three bases does not shift the reading frame. T / F
2.3 A nonsense mutation creates a stop codon and usually shortens the resulting polypeptide. T / F
2.4 Point mutations affect the whole chromosome rather than a small section of DNA. T / F
3. Classify the mutation from a sequence comparison
Each row shows an mRNA original sequence and an mutant sequence read 5′→3′. Vertical bars show the codon reading frame. Identify (a) the type of point mutation (substitution / insertion / deletion) and (b) the codon-level outcome (silent / missense / nonsense / frameshift). Use the simplified codon key under the table. 8 marks (1 each)
Codon key (simplified, mRNA codons 5'→3'):
AUG = Met (Start) UUU/UUC = Phe UCA = Ser UGA / UAA / UAG = STOP
CUG = Leu GAA / GAG = Glu GUG = Val AAA = Lys
CCU = Pro ACA = Thr GCA = Ala CGU = Arg
Row 3.1 Original: AUG | UUU | GAA | CCU | UGA
Mutant: AUG | UUC | GAA | CCU | UGA (one base changed in codon 2)
Row 3.2 Original: AUG | GAA | CCU | CGU | UGA
Mutant: AUG | UAA | CCU | CGU | UGA (one base changed in codon 2)
Row 3.3 Original: AUG | AAA | CCU | GGA | UUU | UGA
Mutant: AUG | AAA | XCC | UGG | AUU | UUG | A... (one extra base X inserted at start of codon 3)
Row 3.4 Original: AUG | CUG | GAA | GUG | AAA | UGA
Mutant: AUG | CUG | GAG | UGA | AA... | UGA (one base deleted from start of codon 3)
| Row | (a) Type of point mutation | (b) Codon-level outcome |
|---|---|---|
| 3.1 | ||
| 3.2 | ||
| 3.3 | ||
| 3.4 |
4. Function and reasoning recall
Answer each in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 Why is a substitution usually less disruptive than a single-base insertion or deletion in the middle of a gene?
4.2 Explain how the degeneracy of the genetic code (more than one codon for the same amino acid) makes silent mutations possible.
4.3 Why does a nonsense mutation usually produce a non-functional protein?
4.4 Why does an insertion or deletion of three bases not cause a frameshift, even though the protein may still be altered?
4.5 Why is a frameshift that occurs near the start of a gene usually more damaging than one near the end?
5. Order the HSC logic chain
The lesson summarises the HSC logic chain for point mutations as: DNA change → codon change → amino-acid sequence change → protein structure/function change → possible phenotype change. The six tiles below are out of order. Write the correct sequence number (1–6) next to each tile, then write one short sentence connecting tile 3 to tile 4. 7 marks (6 for ordering, 1 for the linking sentence)
| Order (1–6) | Tile |
|---|---|
| Phenotype may change (e.g. red blood cell shape, enzyme failure). | |
| One base in DNA is substituted, inserted or deleted. | |
| Codon(s) in the mRNA are altered or regrouped. | |
| Protein structure and/or function may change. | |
| The transcribed mRNA carries the altered sequence. | |
| The amino-acid sequence of the polypeptide may change. |
5.7 Linking sentence — connect codon change to amino-acid sequence change:
Q1 — Term–definition matches
1.1 point mutation • 1.2 substitution • 1.3 insertion • 1.4 deletion • 1.5 frameshift • 1.6 silent mutation • 1.7 missense mutation • 1.8 nonsense mutation • 1.9 codon • 1.10 reading frame.
Marking notes. 1 mark per correct match (max 10). No half-marks; spelling/synonym leniency only where the term is unambiguous.
Q2 — True / false with correction
2.1 False. Correction: many point mutations are silent — the genetic code is degenerate, so a substituted codon may still specify the same amino acid and leave the protein unchanged.
2.2 True. Three bases is one whole codon, so the reading frame downstream is preserved; only one amino acid is added or lost (or changed) rather than every codon downstream.
2.3 True.
2.4 False. Correction: point mutations affect one base or a very small number of bases in the DNA sequence. Whole-chromosome events are chromosomal mutations, covered in Lesson 4.
Marking notes. 1 mark per correct T/F (4 marks). 1 mark per correct correction on a "False" item (4 marks).
Q3 — Classify the mutation
3.1 (a) Substitution; (b) Silent. UUU and UUC both code for phenylalanine, so the amino acid does not change.
3.2 (a) Substitution; (b) Nonsense. The codon GAA (Glu) becomes UAA (STOP), so translation terminates early and the polypeptide is truncated.
3.3 (a) Insertion; (b) Frameshift. One extra base is added at codon 3, so every codon from that point is regrouped and downstream amino acids almost all change.
3.4 (a) Deletion; (b) Frameshift. One base is removed from codon 3, regrouping every downstream codon and in this case generating an early UGA stop in codon 4.
Marking notes. 1 mark per (a); 1 mark per (b); max 8.
Q4.1 — Substitution vs single-base indel
A substitution alters only the single codon it sits in, so at worst it changes one amino acid (or none, if silent). A single-base insertion or deletion shifts the reading frame downstream, so every later codon is regrouped and many amino acids — often all of them after the mutation site — are changed, frequently introducing a premature stop.
Q4.2 — Degeneracy and silent mutations
Because more than one codon can specify the same amino acid (e.g. UUU and UUC both code for phenylalanine), a substitution that changes one codon into a synonymous codon still produces the same amino acid in the polypeptide, so the protein is unchanged. That is what makes the mutation "silent".
Q4.3 — Why nonsense usually disables the protein
A nonsense mutation converts a sense codon into a stop codon (UAA, UAG or UGA), so the ribosome terminates translation early. The resulting polypeptide is truncated, often missing critical functional domains (active site, folding regions), so it cannot fold or function correctly.
Q4.4 — Three-base indels
Three bases form exactly one codon, so adding or removing three bases adds or removes one amino acid without changing the reading frame for the codons downstream. The rest of the protein sequence is intact, even though that one missing or extra amino acid may still disrupt structure or function locally.
Q4.5 — Position of a frameshift
A frameshift near the start of the gene causes almost the entire polypeptide downstream to be wrong, usually producing an early premature stop and a near-completely non-functional protein. A frameshift near the end affects only a few final amino acids, so most of the protein, including its functional core, may still be intact.
Q5 — Logic chain order
Correct order:
- One base in DNA is substituted, inserted or deleted.
- The transcribed mRNA carries the altered sequence.
- Codon(s) in the mRNA are altered or regrouped.
- The amino-acid sequence of the polypeptide may change.
- Protein structure and/or function may change.
- Phenotype may change (e.g. red blood cell shape, enzyme failure).
Linking sentence (3 → 4): When a codon is altered or the reading frame is shifted, the ribosome reads different mRNA triplets during translation, so it inserts different amino acid(s) into the growing polypeptide.
Marking notes. 1 mark per tile in the correct position (6 marks). 1 mark for a linking sentence that explicitly references translation / ribosome reading the altered codon(s) to insert different amino acid(s).